The present invention relates to crosslinking agents prepared from a modified aminoplast resin and to powder coating compositions containing such crosslinking agents.
In recent years, powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content (xe2x80x9cVOCxe2x80x9d), which significantly reduces emissions of volatile organic compounds into the atmosphere during application and curing processes.
Hydroxyl, carboxyl, carbamate and/or epoxy functional resins, such as acrylic and polyester resins having relatively high glass transition temperatures (xe2x80x9cTgxe2x80x9d), are commonly used as main film-forming polymers for these coatings. Because acrylic polymer systems can be more heat-resistant than condensation polymers, they can provide powder coating compositions having improved storage stability. However when exposed to the extreme temperatures which can be encountered during shipping and/or storage in many geographic areas, even better powder coating stability is desired. By xe2x80x9cstorage stabilityxe2x80x9d is meant the ability of the individual powder particles which comprise the powder coating to resist the tendency to adhere to one another, thereby causing xe2x80x9cclumpingxe2x80x9d or xe2x80x9cfusingxe2x80x9d of the powder coating composition upon storage prior to application. Powder coating compositions having very poor storage stability can be difficult, if not impossible, to apply.
Aminoplast resins are well known in the art as low cost crosslinking agents for hydroxyl, carboxyl and/or carbamate functional polymers in conventional liquid coating compositions. Common aminoplast resins are based on condensation products of formaldehyde with an amino- or amido-group carrying substance. Examples of these aminoplast resins include the methylol and alkoxymethyl derivatives of ureas, melamines and benzoguanamines which are most commonly used in liquid coating compositions. Such aminoplast resins provide enhanced coating properties such as exterior durability, chemical resistance and mar resistance.
Attempts to produce powder coating compositions based on conventional aminoplast resins which exhibit these desirable properties have been largely unsatisfactory because these materials are typically in liquid form and, as such, cause poor powder stability.
The methoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although solid in form, these materials nonetheless can depress the Tg of the powder coating composition significantly, even when combined with high Tg film-forming polymers such as the acrylic polymers described above. Such a depression in Tg also can result in poor powder stability.
Moreover, the use of conventional aminoplast resins in powder coating compositions can result in the phenomenon commonly referred to as xe2x80x9cgassingxe2x80x9d. xe2x80x9cGassingxe2x80x9d occurs as a result of vaporization of the alcohol generated in the thermally induced aminoplast crosslinking reaction. The alcohol vapor is driven off through the coating film upon heating and, as the viscosity of the coating increases during the curing process, pinholes or craters are formed as the gas escapes through the coating surface.
Carbamate functional polymers, that is, polymers having reactive pendent and/or terminal carbamate functional groups, are well known in the art as suitable film-forming resins for liquid coating systems where, for example, when combined with an aminoplast curing agent, they provide coatings having excellent acid etch resistance. The carbamate NH groups react readily with the methoxyl groups of the aminoplast resin, thereby forming a urethane linkage which provides this acid etch resistance. These carbamate functional polymers further provide coatings that have excellent durability and adhesion properties.
Copending U.S. patent application Ser. No. 09/538,836 discloses powder coating compositions comprising a solid particulate mixture of a carbamate functional polymer, for example an acrylic, polyester and/or polyurethane polymer, in conjunction with a glycoluril resin. Due to the high glass transition temperature of the carbamate functional polymer, the powder coating compositions provide improved storage stability as well as coatings having excellent acid etch resistance. However, as discussed above, in some powder coating systems, the glycoluril crosslinking agent can depress the Tg sufficiently to adversely affect powder stability.
It would, therefore, be advantageous to provide an aminoplast-based crosslinking agent suitable for use in a powder coating composition which gives a highly stable powder as well as an acid etch resistant coating free of pinholes or crater resulting from xe2x80x9cgassingxe2x80x9d during the curing process.
In accordance with the present invention, an aminoplast-based crosslinking agent is provided, comprising an ungelled reaction product of the following reactants:
(A) at least one aminoplast resin;
(B) a reactive urethane group-containing adduct; and
(C) at least one compound different from (B) having active hydrogen groups reactive with aminoplast resin (A). The compound (C) is selected from at least one of:
(i) compounds having the following structure (I): 
xe2x80x83wherein X is aromatic; R1, R2, and R3 can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R1, R2, and R3 represents an active hydrogen-containing group which is reactive with the aminoplast resin (A); and
(ii) compounds having the following structure (II or III): 
xe2x80x83where Rxe2x80x2 and Rxe2x80x3 are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms; and
(iii) compounds different from both (i) and (ii) and having a melting point of at least 80xc2x0 C.
The reactive urethane group-containing reaction-product adduct (B) comprises a reaction product of (1) at least one mono-isocyanate and (2) at least one polyfunctional polymer having functional groups reactive with the mono-isocyanate (1). The crosslinking agent is essentially free of urethane NH functionality and has a glass transition temperature of at least 25xc2x0 C.
The present invention also provides a method for preparing the crosslinking agent described above. The method comprises the steps of: (1) reacting (a) at least one mono-isocyanate, and (b) at least one polyfunctional polymer having functional groups reactive with the mono-isocyanate (a) in a molar ratio of reactant (a) to reactant (b) ranging from 1: 1.8 to 2.0 to form the reactive urethane group-containing adduct (B) described above; (2) combining the reactants (A), (B) and (C) described above in a ratio of total combined moles of (B) and (C) to moles of aminoplast resin (A) ranging from 1:1.5 to 3.2 to form a reaction admixture; and (3) heating the reaction admixture formed in step (2) to a temperature ranging from 95xc2x0 C. to 135xc2x0 C. for a time sufficient to form a powder crosslinking agent having a glass transition temperature of at least 25xc2x0 C. which is essentially free of urethane NH functionality as determined by infrared spectroscopy.
Further provided is a powder coating composition comprising a solid particulate mixture of a reactive group-containing polymer having a Tg of at least 30xc2x0 C., and the crosslinking agent described immediately above.
The present invention additionally provides multilayer composite coating compositions comprising a base coat deposited from a base coat film-forming composition and a top coat over at least a portion of the base coat. The top coat is deposited from a powder top coating composition comprising a solid particulate film-forming mixture of (A) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30xc2x0 C. and (B) the crosslinking agent described above.
Coated substrates are also provided.